Never, Ever Use a Fieldbus

Never, ever use a fieldbus – just keep using 3-15psig instruments, or better yet go back to manually operated valves.Maybe it’s ok to use 4-2omA instruments, if you are really adventurous, but never, ever use a fieldbus.At least that’s the conclusion you might reach after reading “Crossing the Chasm” at ControlGlobal.com.

It begins by comparing installing a fieldbus to juggling swords.The “first respondent says “in the industry we serve, we do not want it or need it yet.”Fieldbuses he means, not swords.“Overall it sounds like the benefits are offset by the associated headaches,” says another.

Another says that there is a learning curve.I hope this does not shock anyone but there was also a learning curve to go from manual control to automatic control and from 3-15psig to 4-20mA.

An engineer from Wastewater Treatment Division of King County in Washington complains about the lack of uniformity with his network.Please note that he is not talking about PROFIBUS!

Good luck finding a digital fieldbus expert the article says.Actually you don’t need luck; you need this link to PROFIBUS Certified Network Engineers.We offer Certified Network Engineer training in PROFIBUS and PROFINET.Those who pass the rigorous exam are certified and listed on our website.You will find thousands around the world.And if you are planning on installing PROFIBUS or PROFINET we recommend you join their ranks.Classes are offered frequently in the US and around the world at our certified PI Training Centers – PROFI Interface Center in the US.Class details and a link to registration are here.

The article concludes: “Are the benefits of fieldbus worth the costs and risks? It’s your call. Just make sure that you include all of the costs and benefits in your analysis based on good information from unbiased sources.”Also remember that not all fieldbuses are created equal.Costs should include training and tools.Benefits include installation savings, but more importantly, more uptime.(See the ARC White Paper “The Value Proposition of PROFIBUS in the Hybrid Industries”.)

Or you just wait and see – wait while your competitors use fieldbuses to their advantage and see them prosper at your expense.It’s your call.

Is their anything good in this article besides our drawing*?The recommendation to take training is the only one I found and it’s a good suggestion.

“Some of most baffling problems occur during startup” is certainly a true statement.The idea that manual observation may be needed to find some problems has some validity but you should also know that there are many tools available to help find problems on the network.Certainly startup is the time when you will need such tools.

My conclusion: never, ever use a fieldbus…

…unless you want the engineering to be done sooner…

…unless you want the startup to be quicker…

…unless you want maintenance to be easier…

…unless you want more uptime…

…unless you are willing to invest modestly in training.

—

* We provided the drawing in the article showing multiple fieldbuses connected to PROFINET.The caption would lead you to believe this is difficult; it’s not.Here’s my explanation of the drawing:

For many years now process instruments with PROFIBUS PA have been connected to controllers, be they PLCs or DCSs.Because PROFIBUS PA and PROFIBUS DP have the same protocol but different physical layers, instruments and discrete IO have been accessible with the same diagnostic and configuration tools.(PROFIBUS PA uses a Manchester-Encoded, Bus-Powered physical layer that is identical to FF’s.PROFIBUS DP’s physical layer is RS-485.)Before PROFINET, when different fieldbuses had to be connected to a controller, they each used their own interface cards in the controller.For example, if a DCS used FF for the process instruments and DeviceNet for the discrete IO, the DCS needed two different cards… and the buses needed two different sets of diagnostic and configuration tools, each with its own learning curve.PROFIBUS is the exception to this generalization since PROFIBUS PA devices connect to PROFIBUS DP through a link module and PROFIBUS DP connects to the controller.

PROFINET embraces even greater diversity as shown in the diagram.PROFINET uses “proxies” to integrate all the popular fieldbuses.A proxy is similar to a gateway in that both bridge dissimilar networks.A gateway is typically created by an individual supplier that understands both networks and maps them through their gateway.Proxies are part of the PROFINET specification so the mapping is standardized regardless of the supplier.The proxy specifications are the result of a collaborative effort in the PI working groups.In the case of the HART proxy, the HART Communication Foundation (HCF) participated in the working group.

Any fieldbus is an enabler for the connection to the automation system and then to the information systems.PROFINET goes beyond just enabling as it has specified communications to the MES layer for Asset Management information (with specifications for other types of communication in the works).